Battle of the Frigidus

The Battle of the Frigidus occurred from 5 to 6 September 394 along the Frigidus River (now the Vipava River of Slovenia), fought between the armies of the Western Roman emperor Eugenius and the Eastern Roman emperor Theodosius I. The battle, caused by Eugenius' appointment of pagan senators to important ministerial posts, led to the final decline of Hellenic paganism in favor of Christianity over the following century. Theodosius briefly reunited the Roman Empire before his death a year later, and no Roman emperor would ever rule over the empire again.

Background
During the late 4th century AD, conflict arose within the Roman Senate due to the Christianization of the Roman Empire under Emperor Theodosius I, who issued the Edict of Thessalonica in 380, creating the Imperial Roman Church. After gaining power in the West in 392 AD (with the help of the Frankish magister militum Arbogast, who had the previous emperor Valentinian II murdered), Eugenius appointed pagan senators to key positions in the government. Taking umbrage, the eastern emperor Theodosius, a devout Christian, attacked at the Frigidus River in what is now western Slovenia. His army of 30,000 troops was augmented by 20,000 Visigoths under King Alaric I, a Roman ally.

Battle
Eugenius and Arbogast's pagan Western Roman army placed a statue of Zeus on the edge of the battlefield and applied images of Hercules to their banners, returning to the old Roman practice of appealing to the gods for support. On the first day of battle, the pagan Western Roman forces had the upper hand. Theodosius committed his Gothic allies to battle, purposefully diminishing their ranks to lessen their threat to his empire once the battle was done; 10,000 of the Goths (half of their number) died in the attack, which had little gain. After a sleepless night, Theodosius prayed to God for a storm, and a fierce tempest occurred, blowing clouds of dust onto the faces of the Western soldiers. Theodosius' next attack broke Arbogast's lines, and Theodosius won a decisive victory. Eugenius was captured and brought before the emperor, and he was beheaded for treason; Arbogast fled to the Alps, where he committed suicide due to his hopeless situation. Four months later, Theodosius died, and he divided the empire into the Western Roman Empire (ruled by his son Honorius) and the Eastern Roman Empire (ruled by his son Arcadius).